1. Field of the Invention
The present invention relates to a registration method and an apparatus therefor and, more particularly, to a registration method suitable for registration or positioning in a stepper used in the VLSI fabrication process and an apparatus therefor.
2. Description of the Related Art
In recent years, strong demand has arisen for registration techniques in a variety of fields, and their precision levels vary depending on the technical fields. For example, in a stepper used in the VLSI fabrication process, a registration precision of 1 .mu.m or less is required.
A "vibration slit method" is known well as a high-precision registration method. General relative registration between a wafer table and a reticle in a stepper employing this "vibration slit method" will be briefly described below.
In a stepper, a wafer table is generally located below a projection lens, and a reticle having a transfer pattern is located above the projection lens. A reflection fiducial mark serving as a registration reference is formed on the wafer table, and a reflection reticle mark serving as a registration reference is formed on the reticle. Since the projection lens is present between the wafer table and the reticle, in order to register the wafer table and the reticle relative to each other, lighting having the same wavelength as that of exposure light is required. For this purpose, for example, a half mirror is used to illuminate the fiducial mark with the lighting having the same wavelength as that of the exposure light through the projection lens from the above. Light reflected by the fiducial mark and retransmitted through the projection lens is detected by a photodiode arranged above the half mirror. In the "vibration slit method", the center of the light-receiving surface of the photodiode serves as a registration reference position. In this "vibration slit method", a slit board is located in front of the photodiode. When the slit board is vibrated at a predetermined amplitude and a predetermined vibration frequency with respect to the reference position as the center, an output from the photodiode is changed in accordance with a difference between the reference position and the position of a fiducial mark image. When the position of the fiducial mark image coincides with the reference position, a slit vibration frequency component in the output from the photodiode becomes zero. By utilizing this phenomenon, the wafer table is so shifted that the slit vibration frequency component in the output from the photodiode becomes zero, thereby aligning the fiducial mark with the reference position. In this case, the position of the wafer table is measured by a laser interferometer or the like, and the measured value is stored as an origin of the wafer table.
In the same procedures as described above, the reticle mark is aligned with the reference position, the position of the reticle is measured by the laser interferometer or the like, and the measured value is stored as an origin of the reticle. Relative registration between the wafer table and the reticle is performed by the above procedures.
The above "vibration slit method" poses the following problems. More specifically, since registration must be performed while an output from the photodiode is monitored, registration cannot be performed in such a case that a lighting intensity of the lighting varies as a function of time, e.g., a pulsed laser beam must be used as lighting. In order to increase a registration resolution, a magnification of a mark image must be increased. In this case, however, the lighting intensity of the mark image is decreased, and the laser beam cannot be used as lighting, resulting in inconvenience.
A conventional "vibration slit method" which is known to be capable of performing registration at a relatively high precision cannot be used when lighting greatly varies as a function of time or the lighting intensity of the mark image is low.